Method of and apparatus for determining consolidation of fluid concrete masses



Aug. 12, 1952 GLEGG 2,606,748

METHOD OF AND APPARATUS FOR DETERMINING CONSOLIDATION OF FLUID CONCRETE MASSES Filed July 27, 1949 2 SHEETS-SHEET 1 VT INVENroR= Gammon Luvpsny 611E6- & 3y:

WW 2140, d.

Aug. 12, 1952 GLEGG 2,606,748

METHOD OF AND APPARATUS FOR DETERMINING CONSOLIDATION OF FLUID CONCRETE MASSES Filed July 27, 1949 2 SHEETS-SHEET 2 Galvan L lmmsny 61,561-

Patented Aug. 12, f 1952 "UNITED; STATES PATENT OFFICE 1 METHOD OF AND, APPARATUS FOR DETER- .MINING CONSOLIDATION or FLUID CON- oRn'rE MASSES 1 Gordon Lindsay Glegg, Kingston .Hill, England,

assignor of one-half to The .Square Grip Beinfor-cement Company (London) Limited, London, England Application July "27, 1949, Serial No. 107,104 In Great Britain July 28, 1948 2 Claims.

This invention relates tomethods of and means for determining the degree of consolidation of concrete masses when in a fluid condition.

It is well'known in connection with th laying or gravity moulding of masses of concrete "especially when such masses are .deep that special precautions have to be takento eliminate pockets of occluded air, which readily form during the deposition'of the fluid aggregate, since, if

they are allowed to remain, they constitute a serious source of Weakness in the finished concrete structure. A satisfactory method of eliminating any air pockets formed during shovelling or pouring of a wet aggregate is to subject the deposited wet mass to rapid agitation by means of a vibrating appliance. One exampleof such optimum duration of tim for vibratory consolidating treatment of a given intensity. If the time of treatment be too short, having regard to the intensity of the vibration, the occluded air will not be expelled, whereas toolong atreatment will result in the loss of homogeneity referred to.

It is one object of the present invention to provid a method of and means for determining whether or when a vibratory treatment applied to a wet concrete mass has been carried to the optimum'point for the consolidation of such mass.

.According to the present invention a method of producing concree consolidated to the desired degree comprises vibrating wet fluid concrete,

inserting at least one pair of electrodes in the concrete, applying an E. M. F. .across the electrodes, continuing vibration unti1 th current which is caused to flow by the applied E. M. F. reaches a predetermined value, such predetermined current value having regard to the magnitude of the E. M. F., beingthat which indicates that the concrete is consolidated to the desired degree.

For a better understanding of the nature of the invention and th means of carrying it into effect reference will be made to the accompany- .ing' drawings, in which:

Figure :1 is a typical curve showing the relationship between the electrical current passing through a mass of wet concrete undergoingvibra- 'tion, and the vibration time,

I Figure .2 shows diagrammatically one constructional embodiment for carrying out the method of the invention,

Figure .3 shows further curves indicating the relationship between the electrical current passing in a mass of wet concrete undergoing vibrationand the vibration time,

Figure 4 shows diagrammatically another constructional embodiment for carrying out the method of the invention, and V Figure 5 shows three curves indicating the relationship between the compressive strength of different mixes of concrete after setting and the vibration time.

It hasbeen observed that whereas cement water has a relatively high electrical conductivity, the other constituents of a concrete mix (sand and ballast) ar practically non-conductive. It has also been noted that the electrical resistance of a wet concrete mass in the course of a consolidating treatment varies, and further- 'more the variations do not follow a linear law.

'If two electrodes (not being in contact with each other) are submerged in a wet massxof concrete and an E. M. F. is applied across the electrodes, it is found that the resulting current varies with the area of contact between the electrodes and the concrete. When the concrete "has many pockets of occluded air therein the electrodes will be in contact partly with concrete and partly with air, and consequently the current flowing will b comparatively small. As the of vibration VT. It will be seen that the current increases rapidly with increasing vibration time until the point I is reached and thereafter the curve shows that the current becomes practically constant in spite of continued vibration. Since the current does not increase after the point i is reached, the indication is that the area of contact between the electrodes and th wet concrete does not thereafter increase. This situation can only be attained when the concrete is completely consolidated i. e.- when all the pockets of occluded air have been eliminated.

.It will be understood therefore that consolidation can be considered as being complete either vcrete mass.

predetermined magnitud will of course vary 2112-,

cording to the type of concrete mixwhich is used.

By the basis of the phenomenon just indicated it is easy to establish experimentally for any given mix of concrete, and within limits sufflciently close for practical purposes," the current value for a given E. M. F. which corresponds to the optimum degree of consolidation. Thereafter any wet concrete mass of the same composition can be deemed properly and sufficiently consolidated if, on test, it shows for the same applied E. M. F. a current value within the predetermined limits. Furthermore, the current value at any time during vibration is as an indication of the degree of consolidation at that time.

A simple apparatus for carrying out the method of determining the degree of consolidation of concrete is shown in Figure 2 and comprises essentially a pair of electrodes 2 and 3 mounted in a supporting head 4 and mutually insulated, one from the other. The electrodes 2-and 3 are connected in series with a low voltage current source 5 of known E. M. F., and an electrical measuring instrument 6 capable of showing changes of current over the range covered in the course of a vibrating operation.

When two pairs of electrodes are submerged in a wet concrete mass undergoing vibration, one pair being near the top of the mass and the second pair being near the bottom of the mass, characteristic curves of current I against the vibration time VT are of the form indicatedin Figure 3. In Figure 3, the curve 7 corresponds .to the pair of electrodes near the bottom of the concrete mass and the curve 8 corresponds to the pair of electrodes near the top of the con- It will be seen, that, as might be expected, the curve indicates that the concrete near the bottom of the mass consolidates slightly sooner than the concrete at the top.

.When the whole concrete mass is consolidated the two current values are substantially the same except when the mass is completely consolidated,

is utilised in the apparatus shown diagrammatically in Figure 4.

In the apparatus shown in Figure 4 the pair of electrodes 9 and I0 and the pair of electrodes II and I2 are mounted on the outer casing of a vibratory appliance I3, such as that disclosed in British specification No. 588,607. The electrodes 9, l0 and II and I2 consist of metal rings which are inserted in recesses cut in the casing of the vibratory appliance. Insulating material 40 is interposed between the electrodes and the casing of the vibratory appliance l3, so that the said electrodes are insulated from the casing.

'The electrode 9" is connected through a switch 29 to one terminal |4A of an alternating current source I4, and the electrode I2 is connected 'tothe other terminal |4Bof the source I4.

Two resistances I5 and N5, of equal value, are connected together at junction 20, in series across the source 4, and the switch 29. The electrode I0 is connected to the electrode II by a conductor 1. A relay device I 8 is connected between the mid-point IQ of the conductor l1 and the junction 20.

The relay I8 consists of a coil 2| connected across the points l9 and 20, the said coil being wound on an iron core 22. An armature 23 carrying a contact 24 is disposed in the vicinity of the coil. The armature 23 is secured to one end of a flexible metal strip 26, the other end of which is secured to a fixed mounting 25. A spring 21 is attached to the armature and. is mounted so that when the core is magnetised the spring opposes motion of the armature towards the core. When the core 22 is not magnetised the spring 21 holds the contact 24 on a fixed contact 28. The contact 28 is connected to the terminal MB of the alternating current source l4, and the contact 24 is connected to the terminal 36 of an electric bell 38, the terminal 31 of which is connected to the terminal I 4A of the alternating current source l4 over the switch 29.

The apparatus shown in Figure 4 operates in the following manner. The vibratory appliance is inserted substantially vertically in a mass-of wet concrete until both pairs of electrodes are submerged, the left hand end of the appliance as seen in Figure 4 being inserted first. When the vibrator is switched on and the switch 29 is closed, the concrete is vibrated and current flows through the concrete between the electrodes 9 and I0 and the electrodes II and I2. When the current flowing between the electrodes 9 and I0 is different from the current flowing between the electrodes II and i2, current flows in the coil 2| and the armature 23 is attracted towards the coil 22, whereby the electric circuit to the bell 38 is interrupted. Vibration of the concrete by the vibratory appliance I3 continues and eventually the current flowing between the electrodes 9 and I0 and the current flowing between the electrodes and I2 become equal. As has been previously indicated, when this condition arises, the concrete has been consolidated to the optimum amount. When equality of these two currents occurs no current flows in the-coil 2| and the armature, under the action of the spring 21, is moved so that the contact 24 encounters the contact 28. Thisv causes the electric circuit to the bell 38 to be completed and the bell operates indicating that the vibration has been carried out to the required extent forconsolidating the concrete to the optimum amount. The operator then removes the vibrator from the concrete or the vibrator is switched off.

The curves in Figure 5 indicate the result of consolidation experiments carried out utilising the apparatus shown in Figure 4 on three different mixes of concrete. Samples of each mix were vibrated for a measured length of time, each sample being vibrated for a difierent length of time from the other samples of the same mix. After vibration the samples were allowed to set and subsequently their compressive strengths CS were determined. The curves 30, 3| and 32, each corresponding to one of the mixes, were then plotted. The arrows 33, 34 and 35 indicate the times at which the bell 38 sounded. It will be seen that the points occur approximately at the values of the maximum compressive strength CS.

In the apparatus described above, alternating current has been used. Actually, alternating current or direct current can be used.

The method of the invention can be applied in the formation of pre-cast blocks of concrete by pouring concrete into box-like containers and allowing it to set. In such a case the electrodes could be located in recesses on the inside of the walls of the containers and be fixtures therein.

I claim:

1. An apparatus for indicating the progressive consolidation of wet fluid concrete during vibration of the latter to effect removal of occluded air pockets from the concrete and comprising first andsecond pairs of electrodes for insertion into the vibrated concrete at different levels of the latter, an electrical four-armed bridge network having four junctions, a first resistance in a first armof said network, a second resistance equal to said first resistance and interposed in a second armof said bridge network adjacent to said first arm, first conducting means connecting said first pair of electrodes across a third arm of said bridge network, second conducting means connecting said second pair of electrodes across the fourth arm of said bridge network, current responsive means connected between the junction of said first and second arms and the junction of said third and fourth arms, and means for applying an E. M. F. across the junction connecting said first and fourth arms and the junction connecting said second and third arms of the bridge network.

2. A method of consolidating concrete to a desired degree comprising the steps of vibrating wet fluid concrete for the purpose of efiecting removal of occluded air pockets therefrom, inserting two pairs of electrodes in the vibrated concrete at diiferent levels of the latter, applying voltage across each of said pairs of electrodes, measuring the currents flowing through each of said pairs of electrodes, and continuing the vibration of the concrete until the measured currents indicate that the desired degree of consolidation of the concrete has been obtained at both of said different levels thereof.

GORDON LINDSAY GLEGG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,279,143 Patterson Sept. 17, 1918 1,756,286 Farrall et al. Apr. 29, 1930 2,466,453 Locke Apr. 5, 1949 2,468,972 Hagerty May 3, 1949 

